Pollination Biology and Adaptive Radiation of Agavaceae, with Special Emphasis on the Genus Agave
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Aliso 22, pp. 329–344 ᭧ 2006, Rancho Santa Ana Botanic Garden POLLINATION BIOLOGY AND ADAPTIVE RADIATION OF AGAVACEAE, WITH SPECIAL EMPHASIS ON THE GENUS AGAVE MARTHA ROCHA,1,7 SARA V. G OOD-A´ VILA,2 FRANCISCO MOLINA-FREANER,3 HE´ CTOR T. A RITA,1 AMANDA CASTILLO,1 ABISA´Gı ARC´ıA-MENDOZA,4 ARTURO SILVA-MONTELLANO,5 BRANDON S. GAUT,6 VALERIA SOUZA,1 AND LUIS E. EGUIARTE1 1Departamento de Ecologı´a Evolutiva, Instituto de Ecologı´a UNAM, Apartado Postal 70-275, Me´xico, D. F. CP 04510, Mexico; 2Department of Biology, Acadia University, Wolfville, Nova Scotia B0P 1X0, Canada; 3Departamento de Ecologı´a Funcional, Instituto de Ecologı´a UNAM, Apartado Postal 1354, Hermosillo, Sonora CP 83000, Mexico; 4Jardı´n Bota´nico Exterior, Instituto de Biologı´a, UNAM, Apartado Postal 70-614, Me´xico, D. F. CP 04510, Mexico; 5Universidad Auto´noma del Estado de Hidalgo Apartado Postal 69, Pachuca, Hidalgo, CP 42001, Mexico; 6Ecology and Evolutionary Biology, 321 Steinhaus Hall, University of California, Irvine, California 92697, USA 7Corresponding author ([email protected]) ABSTRACT Agavaceae are an American family that comprises nine genera and ca. 300 species distributed in arid and semiarid environments, mainly in Mexico. The family is very successful and displays a wide array of ecological, reproductive, and morphological adaptations. Many of its members play important roles as keystone species, because they produce abundant resources during the reproductive season. In this paper we analyze the current knowledge about the pollination ecology of the different genera in the family and the role that pollination systems have played in the ecological and phylogenetic success of the group. After providing an overview of each of the genera in the family, we discuss in detail aspects of the reproductive ecology of species in the genus Agave s.l., which is composed of ca. 208 species and includes subgenera of Agave (Agave and Littaea), Manfreda, Polianthes, and Prochnyanthes. Finally, we describe the results of analyses to test the hypothesis that there has been an adaptive radiation in the genus Agave. Using chloroplast and nuclear DNA sequences we estimate the age of the Agavaceae family and the genus Agave to be 12–26 millions of years ago (MYA) and 10 MYA, respectively, and show that mean rates of diversification were higher in the genus Agave than the genus Yucca. The values we report for rates of diversification in Agave s.l. are high when compared to other radiations in plants and animals. We suggest that the desertification of North America, which started ca. 15 MYA was critical in the radiation of agaves and that the generalist pollination system of Agave has been more successful in generating new species than the extreme specialization of Yucca. Key words: adaptive radiation, Agavaceae, Agave, bats, Leptonycteris, pollination, reproductive ecol- ogy, Yucca. THE AGAVACEAE FAMILY 1995) have shown that the definition of Agavaceae should be restricted to include the genera and species found in Table Agavaceae are an American family distributed in arid to 1. For a detailed account of Agavaceae and related taxa see semiarid environments; the majority of species are found in Eguiarte et al. (2000). Mexico. Currently nine genera and ca. 300 species are rec- Some other genera in the monocotyledons, such as Hosta ognized in the family (see Bogler et al. 2006 and Table 1). Tratt. (Hostaceae) (Kubitzki 1998) found in Korea, China, All members of the family exhibit a similar basic structure in the arrangement of their rosette, flowers, and inflores- and Japan, and some perennial bulbs from North America, cences. Moreover, they all share a basic chromosome num- such as Chlorogalum Kunth, Hesperocallis A. Gray, and Ca- ber (karyotype), consisting of five large chromosomes and massia Lindl., are now considered to be part of Hyacintha- 25 very small ones. The only exceptions to this karyotype ceae, and are very closely related to Agavaceae. Detailed occur in polyploid species, which have two or more copies taxonomic studies are needed to determine if these genera of the basic haploid chromosome set. should be considered part of Agavaceae or just closely re- Traditional classification systems placed Agave and related lated to the group (see Bogler et al. 2006). genera as part of Amaryllidaceae based on the shared char- Members of Agavaceae display a wide array of ecological, acteristic of the inferior position of the ovaries, while Yucca reproductive, and morphological adaptations to arid environ- and the species related to it were classified as part of Lili- ments. The family has been important for people living in aceae because their ovaries are superior. While the original the Americas since prehistoric times, with various species classification of Agavaceae by Hutchinson (1934) included providing clothes, rope, food, and beverages (both nonal- a wide diversity of species, more recent detailed morpholog- coholic and alcoholic) to humans. Currently, the family is ical (Dahlgren et al. 1985; Alvarez de Zayas 1987; Hernan- of huge economic importance to Mexico because both te- dez 1995) and molecular studies (Eguiarte et al. 1994, 2000; quila and mezcal are produced from Agave plants. In addi- Bogler and Simpson 1995, 1996; Bogler et al. 1995; Eguiarte tion, fibers of significant economic importance are still de- 330 Rocha et al. ALISO Table 1. Genera and species number in Agavaceae. peroyucca whipplei (Torr.) Trel. are only pollinated by Te- geticula and Parategeticula moths (Pellmyr 2003). The Yuc- No. of ca–yucca moth coevolution is considered, along with the No. of species in Genera species Mexico fig–fig wasp interaction, as the premier example of extreme specialization and codependent pollination. Another set of Agave L. 166 125 species, mostly in Agave, is also pollinated during the night Beschorneria Kunth 7 7 by bats, in particular by the genus Leptonycteris (Eguiarte Furcraea Vent. 25 11 Hesperaloe Engelm. 5 5 et al. 2000). However, the bat pollination syndrome is Hesperoyucca (Engelm.) Baker 1 1 ‘‘leaky’’ (that is, open to exploitation and usage by other Manfreda Salisb. 28 27 visitors), since a large number of other animals is usually Polianthes L. 13 13 also involved in pollination (Proctor et al. 1996). Some other Prochnyanthes S. Watson 1 1 Agave species are primarily pollinated by bees, humming- Yucca L. 49 29 birds, and perching birds, in particular orioles (Ornelas et al. Total 293 217 2002). Undoubtedly, pollination ecology has played an important role in the ecological and phylogenetic success of the group, rived from various species of the family, such as henequen but interestingly the most species-rich genera in the family, and sisal from the Agave genus, as well as other fibers from Yucca and Agave, have contrasting reproductive strategies. Yucca and potentially from Hesperaloe. In Yucca, the pollination system is very restrictive (it in- cludes only a set of specialized yucca moths), and involves GENERAL ECOLOGY OF THE FAMILY a shift in rewards, from pollen and nectar to the developing Owing to a suite of morphological and physiological ad- ovules wherein the yucca moths lay their eggs. On the other aptations, members of Agavaceae are especially successful hand, most species in the most diverse genus Agave, with in arid and semiarid environments in the deserts and moun- ca. 166 species, have been selected to produce very large tains of the Americas and play an important role as keystone amounts of nectar and pollen, and are visited by a large species in these habitats because they produce abundant re- coterie of pollinators, ranging from small insects to relatively sources, mainly during the reproductive season, as will be large vertebrates such as perching birds and bats. While discussed below. some species of Agave attract diverse pollinators, it has been While all Agavaceae species form rosettes, there is con- suggested that Agave and bats are also an example of co- siderable variation in the extent of woodiness among species evolution and mutualism (Gentry 1982; Arita and Humphrey such that some are considered to be woody perennials, in 1988). In this paper, we analyze what is known about the particular species in Agave (i.e., A. karwinskii Zucc.), Fur- pollination ecology of the different genera in Agavaceae and craea, and Yucca, while others are completely herbaceous, discuss their evolution in terms of what we know about the producing leaves and inflorescences from a subterraneous phylogeny and evolution of the family using molecular evo- bulb, like all species in Manfreda, Polianthes, and Prochn- lution and statistical tools. yanthes. Most of the species are long-lived succulents, though leaf thickness can vary among species. Perhaps the PHYLOGENY OF THE FAMILY most spectacular adaptation is in their reproductive ecology, as almost all Agave, Furcraea, and Hesperoyucca are mono- Members of Agavaceae have been the object of several carpic (semelparous); the rosette grows for several years, phylogenetic studies, both morphological (Alvarez de Zayas usually more than ten (see Eguiarte et al. 2000, for some 1987; Herna´ndez 1995) and molecular, using either chloro- estimates) and after producing a huge inflorescence, the ro- plast rbcL sequences (Eguiarte et al. 1994; Eguiarte 1995), sette dies. However, though individual rosettes exhibit mon- restriction enzymes analyses of the chloroplast genome ocarpy, many species also reproduce clonally, such that a (Bogler and Simpson 1995), or ITS nuclear sequences (Bog- genet may survive for many generations. Though these three ler and Simpson 1995, 1996; Bogler et al. 1995; Eguiarte et genera are monocarpic, all the other species (the group Stria- al. 2000). The basic relationships among the main groups tae of Agave, Beschorneria, Manfreda, Polianthes, Prochn- have been found to be congruent in the majority of the stud- yanthes, and Yucca) are polycarpic (iteroparous). ies, and are shown in Fig. 1 (see also Bogler et al.